|MIDI and audio|
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Sequencing, the computerized equivalent of tape recording, is a very common and popular MIDI application. Only a few computers have a built-in MIDI connection. Other computers, such as your PC, can hook up to a device called a MIDI interface. This converts the MIDI data into a format the computer can understand and allows the computer to control MIDI instruments.
Sequencing takes advantage of the fact that MIDI data can correlate exactly to a performance on a MIDI instrument. Suppose we feed this performance data to a computer's MIDI in jack, and load a program that instructs the computer to remember the order in which the data appears at the MIDI in jack. The computer acts like a recorder, but instead of recording audio, it stores the digital data that represents the notes you played, and the exact order in which you played them.
If you play a chord, each note in the chord results in a discrete piece of data. These pieces of data, like all MIDI data, are sent one after the other (serially). Fortunately, this happens at a very high rate so that notes played at the same time appear to occur simultaneously, even if a few milliseconds elapse between the first and last notes of the chord.
Once stored in memory, connecting the computer's MIDI out to the instrument's MIDI in recreates the performance. The principle is the same as a player piano, but instead of having keys triggered by holes in a roll of paper, electronic sounds within the instrument are triggered by data contained in the computer's memory. This underscores the importance of the MIDI standard specification. If the computer says play middle C, the sound generator will play middle C, regardless of the manufacturer.
Like a word processor, once the data is in the computer, it can be edited. Notes, phrases, or measures can be erased, altered, transposed, and much more. You can edit as little as the dynamics of one note, or as much as all of the notes in an entire tune.
Each of MIDI's 16 available channels can carry a unique set of MIDI data. Since all of this data travels over one cable, each piece of data includes its appropriate channel ID so that the MIDI receivers can tune in to a particular channel and accept only that data.
A keyboard transmitting over channel 2 will stamp its data as belonging to channel 2. This is particularly useful when sequencing, since each recorded track can be assigned to a unique MIDI channel, and the associated piece of gear can tune into a particular track. For example, if track 1 (set to MIDI channel 1) carries bass and track 2 (set to MIDI channel 2) carries drum data, you would set a bass sound generator to tune in to channel 1 and a drum generator to tune in to channel 2.